Push Rod vs. DOHC engines
 

I'm sure I'm inviting hell with this question:scared0016: but this issue has been bugging me for a while.

First off, I'm engine agnostic. I've owned virtually every engine type and layout on the market. I currently drive a rotary as my toy car, but if you could provide me a hamster wheel that is powerful and efficient I would consider it. I really don't care about the engine design so long as it’s small, lightweight, efficient and powerful.

The new GM Corvette engine is an amazing package in terms of power, weight, reliability and efficiency. Given all that GM can get out of a push rod V8 chocked full of technology, why do Ferrari, BMW, Audi etc... all seem so wedded to DOHC engines in their performance cars that seem to be larger, less efficient, more expensive/complex and physically larger?

Are push rod engines dirtier in terms of emissions? I get that it’s hard to build a small push rod engine (i.e. a I4 replacement), but in terms of larger performance engines, I’m just not seeing the advantage of DOHC.

All engines have advantages and disadvantages, so what are the disadvantages of a high tech (direct injection etc…) push rod engine?

because it works

it's reliable and i you want more power just add displacement

GM is a cheap company that didn't want to innovate since they made their first car, focusing on re-using existing technologies hoping that consumers still think that anything American is the best in the world.

As for efficiency.... Japanese engine builders have been making reliable engines which produce over 1 hp per liter for the last two decades, what has GM made?

I certainly hope it's 'over 1hp per liter'. ;)

100hp/liter?

Good for hauling all us overweight Mericans! I kid I kid.

Sent from my flux capacitor using Taptalk

GM made a lot of money by swindling their customers with outdated technology

then came the big hammer but somehow the laws of capitalism didn't apply, so they took even more money from the people and continued to produce crap

now somewhere somehow someone up in GM management got the idea that maybe it's time they build something that's not shit


torque you can use? yes lets mask all of our inefficiencies with engines that kick people back in their seats, that should keep them docile and satisfied.

herpa derpa

The Oldsmobile based pushrods is rock solid but not an optimal race motor imho

OHV cam-in-block makes a lot of sense if you aren't displacement-limited by racing class or vehicle tax structure.

Obviously, they won't make the same power/displacement as DOHC multivalve engines. But they do make comparable or superior power/physical engine size and power/engine weight.

And for a given power level, larger displacement OHV gives better fuel efficiency than smaller displacement DOHC multivalve.

Also, OHV cam-in-block engines will have a lower center of gravity than DOHC.

There's a certain elegance in the simplicity of large-displacement OHV 2-valve engines. I like 'em so much there's one in place of the rotary in my RX-7 :)

I also like high-revving DOHC multivalve engines, there's one in my S2000 and another in my SV650 :)

It's just a different way of moving valves up and down. Doesn't matter much. The design of the ICE hasn't changed much.

It's all still suck/squeeze/bang/blow.

I know there's a lot of LSx fans in here but to say that higher revving engines of the same power get worse fuel economy is like saying some fatter people eat less because they're trying to lose weight.

The gearbox is the biggest determinant of fuel efficiency, and the higher revving engines have traditionally lacked a true cruising gear. Additionally as you probably know the EPA city test is more a test of gearing than anything else.

An LS3 probably idles at 0.75 gallons per hour or something like that. 0.75 gallons per hour is the fuel flow rate required to keep a typical car moving at 35mph. Think about that for one second.

I'd be willing to bet that E90 M3s get the same mileage as Camaro SS in the real world, probably better. No, I have not looked online to see what mpg people are actually getting.

What I think the big American V8s should be admired for is how they manage to be so light and pack so much displacement. As @LSxJunkie points out, an LS3 is lighter than an S65, which is already extremely light (the S54 was heavier). GM managed to make a 6.2L lighter than a 3.2L BMW engine. That is impressive. However, you definitely cannot argue that they are fuel efficient. Around town they guzzle gas like nothing else, but on the highway they are okay since GM is willing to suck up their pride and widen gear ratios, whereas BMW insists on short close ratios, same with the Japanese manufacturers. Of course, part of that is the "broad powerband", aka early torque peak.

haha okay, because car buyers are 100% informed consumers right?

If people actually knew what they were getting from GM then GM would have been out of business a long time ago with the carolla being a staple econobox of every american household.

Ford tried the OHC route in the 60's with the 427 sohc.It made better than 100+hp over the best push rod boss 429 of its day,but was expensive to produce.

Instead of GM spending a ton on new technology,they spent a little here and there refining the old.The one thing i can give them credit on is keeping the corvettes heritage in tact.The new vettes still feel and sound the same way they did back in the day because of that pushrod motor.

4/5 valves per cylinder will always be more efficient than 2. Take any DOHC engines and remove 2 valves, and try to keep the same Horsepower, good luck. You need more displacement for offsetting those drawback. As far as I know, pushrod engines are limited to 1 intake and 1 exhaust valve. It's easy to see why DOHC engines are so popular, you get get alot of horsepower(and higher RPM) more easily out of them at the added cost of weight (Which isn't a big deal at all).

It doesn't matter though, those pushrod GM is pumping out are nice engines, they are just not impressive as far as engineering goes. There's nothing better than the other. :happy0180:

Just to add some perspective here on the old vs new engine tech argument:

The first production Dual Over Head Cam engine was in 1924.

The first production Over Head Valve engine using pushrods like the corvette engine was 1949.

So technically, today's pushrod engine is newer tech. Not that it really matters. we're talking about the 20s and 40s here. So the "higher tech" argument is pointless, and actually rather silly.

Depends on what you mean by "efficiency". Normally, it would be BSFC, brake specific fuel consumption.

The most efficient designs in terms of BSFC won't be the most power-efficient in terms of power/displacement, that's for sure.

Fewer valves and fewer intake tracts will generally give better BSFC, as for a given flow area/volume, you will have less of the flow near the tract walls. Reduced losses drawing the charge into the intake.

Also, more cams, chains, and valves => marginally greater frictional losses.

You could do a multivalve OHV engine.

Weight is a very big deal. So is size. For a given size/weight, you can make as much or more power with an OHV V8 vs. a DOHC V8 (which would have to be of MUCH smaller displacement to be the same size/weight).

Only if you judge "engineering" by counting valves and camshafts. The engine engineering at GM is top-notch.

There's more than one way to skin a cat. DOHC/multivalve is required to get maximum performance out of limited displacement. If there is no displacement limit, the simpler, lighter, smaller, cheaper OHV/2vpc design of greater displacement is as good an engineering solution (superior on some fronts) to achieving the same power/performance.

To disparage the LS-series V8s because of their lack of valves and camshafts is to be too fixated on the means, rather than on the end result.

All said, I'm a big fan of diversity, and LOVE higher-revving DOHC multivalve engines as well.

But when it comes to figuring what's the best engine to get maximum power in a small, lightweight package, the large-displacement OHV V8 is very hard to beat...

I have a hard time believing the "size" and "weight" argument is always valid as far as pushrods engines goes.

Considering motorcycles greatly benefit of smaller size and weight, I can't beleive one could take an inline 4, converted it to pushrod / 2 valves per cylinder, up the displacement and get a better engine still pushing almost 200 hp. It would be noisy/rattly and couldn't rev for crap.

Buell's V-twin never came close to what ducati or even the japanese did with their dohc engines.


Case in point, each technology has its ups and down, and the specific vehicle they go in is probably the most important factor about what to use.

We all know inline 6s are inherently heavier due to the block design... just look at 1JZ, 2JZ, RB20, RB25, RB26, M50, M54, S50. all very similar weight.

4.6 liter DOHC Ford mod motor vs. 5.0 liter OHV engine it replaced:
http://i21.photobucket.com/albums/b279/einarak/32v.jpg

4.0 liter DOHC Toyota 1UZ V8 vs. 5.7 liter LS1 (same external dimensions as current 6.2):
http://sphotos-b.xx.fbcdn.net/hphoto...62609930_n.jpg

Benefits of OHV cam-in-block design are pretty much limited to V-engine layouts, where there's a big valley with plenty of room for a camshaft and pushrods. Inline engines definitely want to be SOHC or DOHC.

Presumably talking about the old air-cooled H-D twin and not the later Rotax mill (which was DOHC/4vpc). Suffice it to say, the very long-stroke air-cooled Harley engine was never by any stretch designed to be a performance engine. Not really a valid comparison.

LS3 vs. S65 is a MUCH more reasonable comparison.

Yup. I'm not averse to any engine architecture that gives me the power I want for as little weight as possible.
The LS V8 does lend itself to being swapped into smaller/lighter-weight cars much more readily than physically bigger DOHC V-engines.

And thanks to Camaro SS sales (nothing but donor cars to me!), there will be a good supply of LS3s and T6060 transmissions for some time to come :)

H-D air-cooled Vtwin not remotely a performance engine as previously noted. But it should be pointed out that the smaller/lighter H-Ds with those engines get WAY better fuel economy than similar size/weight Japanese bikes with DOHC/multivalve engines!
(and way way WAY better vs. sportbikes).

ScionRacer, I'm not picking on you directly but want to speak to phrases like this, "heritage".

When I hear things involving cars and car companies and terms like heritage or tradition, I kind of just check out. Usually when I'm hearing those words it's to disguise a company's lack of innovation, lack of imagination and desire to turn a profit by continually fooling the public into buying a "new thing" that's not actually new; it's just the same-ol' same-ol' wearing a new dress and new makeup. This just isn't a Pushrod vs DOHC debate, but my disappointment with American manufacturers in general.

And here's why.... Nothing by American companies gets done "just cuz" or "just for the sake of". It only gets done for profit. (Unfair and partially untrue over generalization used for making a point) Now living in America, we all think this makes sense, this is "normal". Well, it's normal to us and it also explains a lot. Other countries have a culture where things are done "just because", sometimes.

Examples:
Why is Ferrari... well... Ferrari? Because Enzo didn't make cars for profit... profit wasn't the goal. Ferrari only made street cars to fund grand prix racing and that's pretty much how their game is still played with obviously some changes but...

Another example is found from Toyota in this article:
http://wot.motortrend.com/toyota-gt86-engineer-upbeat-about-potential-supra-and-mr2-successors-291229.html

and

Show me an American car company with the fiscal balls to put anything like that in print!


So anyway, back to pushrods... at this point the types of technology than can be utilized in cars today is outstanding and Godbless GM for trying with the VOLT (and thank you taxpayers) but when it comes to the Charger, the Vette, the Mustang and the Camaro... It's all just selling the consumer.. "heritage" and "tradition" without ever having to really innovate, cuz what's more 'Mericun than Pushrods! :sigh:

I have a SV650s sports bike V-twin that averages almost 50 MPG. Are harleys THAT much better?

The best I ever heard of a Harley getting good economy was with the half engined Buell Blast which got crazy MPG.

http://www.fuelly.com/driver/noteczuki/sv650s
http://www.fuelly.com/motorcycle/buell/blast

Wait a minute, so there are definitely packaging advantages for OHV V engines, but what about weight? I was under the impression that these OHV engines have a single camshaft driving all the cylinders? In that case I guess a more fair comparison would be SOHC vs. OHV I guess? Neither has cam phasing ability that way.

I think perhaps the issue is that the benefits that DOHC has for higher revs is wasted on engines that don't rev any higher than pushrod engines.

Cam phasing is way more limited on single cam motors (OHC or cam in block) since there is no overlap/LSA control.

It's something to note that for all GM's continuous work on the SBC, they don't use it in small motors.

Lap times are not a measurement of manufacturers culture of innovation. You don't really need a great deal of innovation to go around Laguna or the Ring very fast. Neither of those lap times by either car suggests anything "innovative" in their design.

I also don't think they're necessarily resting on their laurels. I said they were "selling heritage" which is different.
So yeah, I don't think they just sit back and never change but I do feel they often rehash the same ingredients, slap on a familiar label and sell it as "Now you can own this iconic piece of history in this new 2013 model!" Bleh.

Chrysler did OHV cam phasing differently. In the Viper V10, there is a camshaft inside of a camshaft, allowing the intake and exhaust lobes to move independently of each other, allowing the computer to adjust LSA.

Innovation vs. Cost vs. Reliability vs. Results. You don't need the first to achieve the three latter, and it's just as innovative to be able to do more with less instead of throwing money at innovation for the sake of innovation.

And it's not like GM doesn't do innovation. GM invented magnetorheological dampers in the late 90s and were first seen on the Cadillac STS, followed by the C5 Corvette. Since then, GM has licensed magnetic shocks to pretty much everyone, including Ferrari. They were also the first to sell a car with a Heads Up Display(1988) and automatically dimming high-beams (Autronic Eye, 1952).


Also, lest everyone forget, GM also had a line of DOHC small displacement V8s. Anyone remember these?
http://caddyinfo.com/wordpress/wp-co...lr-v_sts-v.jpg

Those two cars had 4.4L DOHC motors with roots blowers (because torque.) The STS-V made 469/439, but only wound out to 6700rpm. The XLR-V made 443/414. This line of motors was killed because the take rate was low, the cost to continue development was high, and nobody wants a DOHC Corvette because they're a bitch to make cheap power with.

That brings me to another point.

How much money does it cost to take a BMW S65 and make a safe 540hp that you can hammer during 30 minute sessions? Somewhere around $20k, if I recall correctly.

In an LS3, that's a cam, headers, valve springs, and a tune. All told, $5k with labor. And there is no need to run aftermarket piggyback fuel management because GM Powertrain Control Modules (PCMs) are completely unlocked. All you need is a cable and EFI Live or HP Tuners (plus credits) and you have complete access to fuel maps, spark tables, timing tables, and everything else in the PCM. If you want to make relatively cheap, reliable power, there is no other way to do it.

Well to be fair, the S65's ITBs are probably very difficult to control, but it's not like you can argue ITB=worse for that reason. If they wanted they could've made it simpler, but then it wouldn't be as neat of an engine. The cheapest method of adding power is always bigger displacement, but just doing that doesn't get us anywhere in terms of technology.

Also, the weight you quoted for the S65 might be a start stop equipped S65, which adds like 10 pounds or something, and I think start stop motors are considered part of the engine at least for quoting service weight.

If Germans were more liberal about adding displacement (well rather, if BMW were, since Mercedes doesn't seem to have a problem with sticking 6+ liter engines into cars) I think the S65 could be a close to 500hp stock engine.

Gm dosent liscence magnetic suspention to anybody. Delphi the gm division responsible for the technology was sold to the damn red Chinese.

Damn those are SMALL... or is it the other ones that are BIG ?

EDIT: So, is GM the only company doing pushrods engines for cars now?

The Ferrari 599, Audi R8, and current Audi TT were all released in 2006. They all feature MagneRide. The Ferrari California was released in 2008. Delphi was sold to BeijingWest Industries in 2009. GM owned the patent when it was licensed out.

And Chrysler in the new HEMI (5.7-6.1-6.4) and 8.4L Viper V10.

I think the only thing GM has done recently that was "bold" was fly their private jets down to their meeting with congress to ask them for their bailout. That was pretty bold if you ask me.

Edit: that was a lowblow, I'll admit.


These pictures Zcan posted of the comparisons of V8's was terrific. That's excellent argument for pushrods if I've ever seen one.

I always figured GM kept the push rod in their flagship for the same reason Porsche keeps rear engines their flagship. It gives their customers that warm fuzzy feeling.

Someone just posted about their old ZR1.

A side by side if the LT1 and whatever the DOHC version is called would be interesting. Back in the day that made 4 valves look like they dominated OHVs.

wow this thread. GM makes some amazing pushrod engines and if you look at how much these engines have improved over the years it makes the "selling outdated tech" argument invalid.

There is a good argument to made that dohc engines will get better fuel economy in theory, but as far as NA hp per the weight and total size of the engine GM's pushrod v8's are pretty damn good.

Well in theory the DOHC design should get you more power per unit mass since the valves no longer become as much of a limiting factor and as we all know short stroke gives more rpms and more power. It's just that the few engines revving past the point where pushrods become a problem don't rev that much higher so they don't fully utilize the DOHC in some sense.

DOHC will get you more power per unit *displacement*, not necessarily more per unit mass. For the same engine *mass*, for V-configured engines my money is on OHV cam-in-block over DOHC.

For the same engine *mass*, you can have a much much larger-displacement OHV engine, as DOHC adds a lot of mass (and physical size).

Actually, there are very good arguments to be made that OHV get better fuel economy. Reduced frictional losses with OHV vs. DOHC, and reduced viscous flow losses drawing air past/through fewer, bigger valves.

The bigger, heavier, less aerodynamic Camaro SS gets better mileage than the smaller, lighter, more aerodynamic M3 with the same power.

Buicks used to get amazing mileage for big/heavy luxury cars with OHV V6s. Mileage went down when they went DOHC.

Me...

I'm very partial to my old DOHC LT5. My motor is an ex-World Challenge built w/ slightly higher duration cams & 368ci (vs stock 350 sitting in the garage).

It pulls like a freight train to the 7550rpm cutoff. Most cars I've had started laying down while approaching redline--not this one... I have to watch the tach like a hawk, or I'll smack the limiter because it's still @ peak HP.
I've had several C5 & C6 Z06s & they've all been sold & my old ZR is still standing after 18yrs...

It is much larger/heavier than the LSX & new LT variants coming out in the C7, no doubt; but nothing sounds like that WOT pull to redline like an LT5 ;)

The valve thing is completely wrong. There is no question that 4 valves per cylinder dominate 2 vpc in pretty much every practical category.

The only area where single intake valves out perform dual is at flow at 0.25D lift rate (when the valve head is theoretically no longer an obstruction) with the same port area. Basically for the same un obstructed flow area, a single large port will outflow 2 smaller ports with the same combined area as the big single. This comes from turbulence between the two air streams, and ends up with around 90-95% of the single.

However, there are a few catches. First is that the 2v will need more cam lift to reach the same .25D since the valve is much larger. This means it takes slightly longer to reach max flow, is harder on the valvetrain, and can compromise compression. Second is the 4V heads dominate at low lift since in that situation, when the valve is still the primary obstruction, it is lift x valve circumference that is the flow area, rather than port (minus stem). This gives 4V much better midrange for the same max power. It sounds strange, but there are few modern situations where we can directly compare them on motors of the same displacement. Third is combustion chamber shape. The layout of a pent-roof 4V allows better flame propagation/efficient burn from the same compression ratio, which means either more power or less fuel.

So in short the OHV's advantages are not from 2V heads, but from its cost and packaging.

Of course they won't be comparable for engines of the same displacement.

For V-engines of the same cylinder count, 2v OHV will generally be comparable to or superior to 4v DOHC in terms of power/engine size (external dimensional engine size, not displacement), power/engine weight, and fuel economy for the same power levels.

Comparing performance engines of the same power level and engine size/weight, the larger-displacement OHV will generally more than make up for any burn efficiency advantage by having lower frictional and pumping losses operating at the lower rpm at which it makes the same cruise power.

Cost and packaging are of course tremendous advantages as well...

For fixed displacement, no doubt DOHC 4v pentroof is the way to go. But if you're not displacement limited...